Humans can imagine possibilities.Humans can imagine objects and materials being
brought into existence that had not existed before.Humans can imagine a possible state of
existence, a state of matter, beyond what they can see or touch.

At the beginning of the 18th Century the English chemist John
Dalton presented an atomic theory.Drawing on previous research he concluded, firstly, that all gases
consisted of tiny particles; and then that all matter consisted of such tiny
particles, which he referred to as atoms (meaning 'indivisible').He became convinced that each element was
made up of different kinds of atom; and that it was the weight of their atoms
that distinguished one element from the other.1

Not everyone accepted his ideas.It took over fifty years for his ideas to become a standard part of
thinking about the elements.2It was not until the early part of
the 20th Century that definite proof of the existence of atoms was
found.By the latter part of the 20th
Century Richard Feynman could assert that "everything is made of atoms".3

Richard Feynman asked how we know that there are atoms.He answered that we know firstly by
developing a hypothesis that there are atoms and then by conducting
experiments that produce results, one after the other, that confirm the
existence of atoms.4

But a theory developed to explain data provided by experiment
and research is likely to begin with a "what if ...?"; a
speculation about how things / the world might
be

The idea that matter was composed of tiny particles moving
around in the void was put forward first in Ancient Greece in the 5th Century
B.C. by Leucippus.Then it was
developed by Democritus and taken up later by Epicurus in the 4th Century
B.C.(Although the idea was rejected
by Aristotle.5

The ancient Greek theory of the atom has been dismissed as pure
speculation not supported by experimental research: it was seen as
representing simple intuition.6 But it was revived in the 17th
Century by Pierre Gassendi.Gassendi
and others at that time could accept the idea of the void in which (he
argued) atoms moved because by that time experimental evidence for the
existence of the void had been identified.7

John Dalton recognised the similarity between his theory and
that of Leucippus and Democritus, so he used Democritus' term 'atom' (meaning
indivisible).Although later research
highlighted a complex inner structure for the atom, it didn't change the
reality of the atom as the fundamental unit of matter and the elements, as
Leucippus and Democritus had proposed.8

Although Leucippus and Democritus did not have the tools to explore
the idea as later experimental scientists did, they could think about the
world which they encountered.They
could speculate about why it was the way it was, and imagine different states
of existence from those which they could first see and touch.The atomic theory of Leucippus and
Democritus was perhaps the greatest feat of pure imagination in the history
of human thought.